Orangutans find home in degraded forests

A new study demonstrates that orangutans can survive in human-impacted landscapes under the right conditions, underscoring the need for protections that go beyond primary forest.

Article by John C. Cannon.

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Scientists and conservationists have long believed that the rapid loss of Southeast Asia’s old-growth, “primary” rainforests in recent decades would leave one of its most iconic inhabitants, the orangutan, with no place to go. The evidence seemed clear: In places where people take over the rainforest, orangutan populations tend to drop off.

But new research on the island of Borneo demonstrates that orangutans can survive in human-altered rainforests, pointing to the underappreciated value of these “degraded” lands.

“If species like orangutans can survive in degraded forests, this means that, even after the forest is degraded, it is still worth protecting,” said Marc Ancrenaz, a biologist and the director at the orangutan-focused conservation organization HUTAN. He and his colleagues published their findings online Tuesday with the journal Proceedings of the National Academy of Sciences.

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A young Bornean orangutan in Sabah. Photo by John C. Cannon.

Ancrenaz has been studying the Critically Endangered Bornean orangutan (Pongo pygmaeus) in the Malaysian state of Sabah for two decades along the banks of the Lower Kinabatangan River, an area that bears many of the signs of human use. Its forests are dotted with human settlements. They have been logged – in some cases multiple times – and oil palm plantations dominate much of the landscape. And yet, a sizeable population of orangutans in the region has persisted.

To understand how these animals have survived the changes from the dense rainforests that once covered much of the island to what exists now, Ancrenaz partnered with researchers from the Carnegie Airborne Observatory (CAO) at Stanford University in California.

CAO ecologist Greg Asner and his team mapped the three-dimensional composition of the rainforest using LiDAR from their specially designed airplane. Short for “light detection and ranging,” LiDAR uses the time it takes laser pulses to reflect back to determine such aspects of the forest as the height of the trees and the shape and size of the canopy, providing a window into the underlying structure of the forest.

Then, they paired that information with three years of on-the-ground observations of orangutans around the Lower Kinabatangan River by Ancrenaz’s team in Sabah.

“We figured out that the orangutans are using the better parts” of degraded forests, Asner said in an interview. They prefer “tall trees and lots of canopy cover,” he added, “meaning the more mature individual trees that are out there.”

Those preferences indicate that not all degraded forests are suitable for orangutans. But if they provide enough food, adequate resources to build their nests (which they do on a daily basis), and the ability to move from tree to tree through the canopy, these forests can be viable habitats.

“Orangutans are smart animals,” Ancrenaz said. “They are great apes, so of course they are able to adapt their behavior,” at least over the relatively short period amount of time they’ve been closely studied.

As a “slow-breeding species,” he added, “It is still quite early to say that degraded forests can sustain viable orangutans over the long term.” But the signs are promising.

Although they seem to prefer interlocking canopies that allow them to move from tree to tree, orangutans will descend to the forest floor to reach another tree in search of food or branches for their nests. That behavior does, however, come at a cost: It takes more energy, and the stroll makes them vulnerable to predators and diseases that can’t reach usually them in the treetops.

Field observations have also revealed that orangutans change their diets to suit the mix of plants that colonize degraded or razed forests.

“This doesn’t mean that primary forests should be exploited,” Ancrenaz said. “We still need to protect primary forests because there are a lot of other species that cannot really adjust to habitat degradation.”

But in a place like Sabah, a lot of the wildlife has little choice but to make their homes in degraded forests that have been logged, or areas that have been cleared completely and are growing back – what are known as secondary forests.

“We definitely need to protect primary forest wherever possible,” Asner said, “but for a lot of regions, if it’s primary, it’s already in a park or some other protected area.”

That’s especially true for Sabah. Two large banks of protected primary forest remain in Danum Valley Conservation Area and Maliau Basin Conservation Area. Beyond that, the state is mostly a mosaic of oil palm plantations, areas set aside for timber and agriculture, and spots where the rainforest is beginning to come back.

Now, Asner said, “The Sabah Forestry Department wants to know, what other forests can we protect?”

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More than 70 percent of Borean orangutan (Pongo pygmaeus) live in human-altered or fragmented forests. Photo by Rhett A. Butler.

This study is part of a larger project led by Asner and the Forestry Department to figure out the best areas for conservation in Sabah. With the data collected from the CAO aircraft, they can also identify the spots with the highest carbon stocks and the most plant species.

By combining those maps with information about the habitats of key animal species, such as orangutans in this instance, they can say to the Forestry Department, “Take these. These are the ones where things are already working well enough for the most species,” Asner said. “Let them regrow and become mature forest.”

The effort aims to safeguard about 400,000 hectares (approximately 1 million acres) of previously unprotected land in Sabah. It will allow the forests to come back and provide a more robust habitat – not just for orangutans, but many other animals. And it will address perhaps the greatest threat that orangutans currently face.

When people move into forests, they’ll often hunt the animals found there, in addition to altering the habitats. When that happens to animals like orangutans, Ancrenaz said, “Of course they are wiped out.”

That’s one reason scientists have suspected that secondary or degraded forests couldn’t support orangutans. But it turns out that it’s one of the associated impacts – more so than the changes to the habitat itself – that’s most responsible for sending their numbers tumbling.

The team’s research shows that orangutans are flexible enough to adjust to recovering forests. “What they cannot adapt to is hunting,” Ancrenaz said.